It is a main duty of the state.

People often misled the foundation of the state this day. The reason we need a government is not for job, for education, or healthcare program (well perhaps we do, but to the lesser extent), the first reason for the state to come in to existence is an ability to monopolize violence, to ensure that the state is the one an only one to use violence within certain territory, failure to do so would result in a failed state of widespread violence and power struggle.

The state is therefore required to use every means necessary within its power to ensure their monopoly of forces. If monitoring its citizen is include, so be it.

Government cannot do the job we have tasked them with without monitoring us to some extent.

The government is responsible for maintaining public order and safety. The only reasonable method of doing this is by providing incentive for individuals to be responsible and respectful. If the government cannot monitor us then it is virtually impossible for them to discharge this responsibility since they have no idea who to reward or punish.

It is government's responsibility to do what is listed in the Constitution, nothing more.

The government's job is to get out of the way, not to monitor the people. Giving the government the task of monitoring so many people is not only near-impossible, but it also gives way to a police state! Assuming that government must watch the people at all times implies that the government has a responsibility towards the people, when the people only have a responsibility to themselves. It is not government's job to protect you, but instead it is government's job to protect your liberty. The best way to protect the citizen's liberty is to get out of the way, and mind your own business!